Optical element and method for producing the same

ABSTRACT

An optical element is provided that includes a base substrate, a waveguide substrate, and a thin film layer that is provided between the base substrate and the waveguide substrate and that has a single-layer structure of a multilayer structure including a film containing Ta 2 O 5  or Nb 2 O 5  as a principal component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of application Ser. No. 10/448,608, filedMay 29, 2003, which application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical element formed with a bondedpair of substrates, and a method for producing the optical element.

2. Related Background Art

In an optical element formed by bonding two substrates, a ridge opticalwaveguide can be formed by forming a ridge structure after thinning oneof the substrates. For bonding these substrates, the direct bondingtechnique is known as a technique for firmly bonding these substrateswithout using an adhesive or the like. The direct bonding allows variousmaterials such as glass, semiconductors, ferroelectrics, piezoelectricceramics, etc. to be bonded with high precision, and therefore, theapplication of the same to optical elements has been highly expected. Asan example of an optical element with use of a pair of directly bondedsubstrates (such a pair hereinafter sometimes is referred to as adirect-bond substrate) such as dielectric substrates, semiconductorsubstrates, and glass substrates, an optical waveguide-type element hasbeen proposed. For instance, JP2574594 and JP06-222229A disclose amethod for forming an optical waveguide by directly bonding lithiumniobate or lithium tantalate as a ferroelectric crystal substrate with asubstrate of the same type or a glass substrate.

Further, several proposals have been made regarding an optical elementformed by bonding two substrates with a thin film interposedtherebetween. In an optical element in which two substrates are used andone of them functions as a waveguide layer, the substrate functioning asthe waveguide layer is required to have a higher refractive index.Therefore, a thin film having a lower refractive index than that of thewaveguide layer is provided between the substrates, whereby light isguided irrespective of the refractive indices of the substrates. Forinstance, JP2574594 and JP06-222229A mentioned above disclose the use ofSiO₂ or SiN as a material for the thin film. Further, JP2574606discloses the use of low-melting glass as the thin film material.JP06-289347A discloses the use of a metal oxide or the like as the thinfilm material.

As described above, the optical element formed with substrates of thesame type having equal refractive indices without a thin film layerinterposed therebetween cannot be used as an optical waveguide. Further,in the case where two substrates having different refractive indices arebonded directly, as in the case where a lithium niobate substrate and aMg-doped lithium niobate substrate are bonded directly, it is impossibleto form an optical waveguide in a substrate having the lower refractiveindex.

By providing a thin film between two substrates, the foregoing problemscan be solved. However, it is difficult to provide a thin film betweentwo substrates. As described in JP2574594 and JP06-222229A, it isdifficult to control a surface roughness of a thin film layer in thecase where SiO₂, for instance, is used for forming a thin film, and athin film formed by sputtering or vapor deposition has a significantroughness on its surface. A film with such a surface roughness is notsuitable for direct bonding. The surface roughness can be reduced by,for instance, forming a thin film using a CVD (chemical vapordeposition) device, but the CVD device is expensive and bulky.Furthermore, the contact between a thin film and a substrate and thebond strength have a non-homogeneous distribution depending onconditions for the thin film formation, and when a bonded substrate pairis subjected to machining, a sufficient strength against the machiningcannot be achieved.

Additionally, as shown in JP2574606, in the case where a low-meltingglass is used for forming a thin film layer, for instance, a low-meltingglass material is applied in a paste form over a substrate, bonded withanother substrate, and subsequently baked. Therefore, it is difficult tocontrol the film thickness so as to achieve a uniform thickness.Further, a technique described in JP06-289347A lacks practical utility,since metal oxide materials to be used for forming a thin film are notdisclosed specifically in the publication.

Furthermore, for controlling a height of an optical waveguide anduniformity of the height thereof in forming an optical waveguide in adirect-bond substrate, it is important to determine a thickness of asubstrate in which the optical waveguide is formed, among the substratesdirectly bonded, and to determine uniformity of the foregoing thickness.However, generally, it is difficult to determine optically the thicknessuniformity of the optical waveguide with respect to a substrate wherethe waveguide is formed, and the control relies on the determination ofa thickness of the entirety of the direct-bond substrate. Therefore,there is a drawback of insufficient thickness uniformity of the opticalwaveguide.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide anoptical element having a structure obtained by bonding substrates, thatis readily producible and provides a high selectivity of substrates usedtherein. It also is an object of the present invention to provide amethod for producing the foregoing optical element.

An optical element of the present invention includes a base substrate, awaveguide substrate, and a thin film layer provided between the basesubstrate and the waveguide substrate, having a single-layer structureor a multilayer structure including a film containing Ta₂O₅ or Nb₂O₅ asa principal component. Therefore, a thin film having high thicknesscontrol precision and small surface roughness can be formed. Further,optical waveguide characteristics can be enhanced, irrespective ofrefractive indices of the waveguide substrate and the base substrate.

Further, it is preferable that at least one of bonding between the basesubstrate and the thin film layer and bonding between the waveguidesubstrate and the thin film layer is direct bonding. This enablesbonding with high precision.

Still further, it is preferable that the waveguide substrate is made ofLiNb_(x)Ta_((1-x))O₃ (0≦x≦1). Therefore, the waveguide substrate has ahigh transmittance in a band of guided light. Moreover, since it has anon-linear optical effect, it can be used in a wavelength converter orthe like.

Still further, it is preferable that an optical waveguide is formed inthe waveguide substrate. This provides an optical element of an opticalwaveguide type.

Still further, it is preferable that the thin film layer has a thicknessof not less than 50 nm. This makes it possible to achieve enhancedwaveguide characteristics without waveguide losses.

Still further, it is preferable that the thin film layer includes a filmcontaining Ta₂O₅ or Nb₂O₅ as a principal component on a surface to besubjected to the direct bonding. This reduces a surface roughness of thebonding surface and enables the direct bonding.

Still further, it is preferable that the thin film layer is a filmformed on either the base substrate or the waveguide substrate in anatmosphere at a temperature of not lower than 100° C. This reinforcesthe adhesion between the waveguide substrate and the thin film layer.

Still further, it is preferable that the thin film layer is a multilayerfilm including a metal layer, the metal layer being not arranged on asurface of the thin film layer on a side of the waveguide substrate, andthe waveguide substrate is bonded with the thin film layer. This allowsthe thickness uniformity measurement by the interference fringeobservation to be carried out readily with respect to the waveguidesubstrate. It should be noted that the interference fringe observingmethod is a method for determining the thickness uniformity of asubstrate by observing a state of interference between reflected lights.

Still further, it is preferable that the metal layer is formed on asurface of thin film layer on a side of the base substrate. This allowsthe thickness uniformity measurement by the interference fringeobservation to be carried out readily with respect to the waveguidesubstrate. This particularly facilitates the measurement in the casewhere the thin film layer is composed of two layers.

Still further, it is preferable that a surface of the metal layer on aside of the waveguide substrate and a surface of the waveguide substrateon a side of the metal layer are separated by not less than 50 nm. Thismakes it possible to achieve enhanced waveguide characteristics withoutwaveguide losses.

Still further, it is preferable that a distance between the metal layerand a light-incident end surface exceeds 1 μm. This makes it possible toachieve enhanced waveguide characteristics without waveguide losses.

Still further, it is preferable that the waveguide substrate is made ofa crystal that is transparent with respect to light having a wavelengthλ and that has a refractive index n₁ with respect to the light havingthe wavelength λ. The base substrate is made of a material having arefractive index n₂ with respect to the light having the wavelength λ.The thin film layer is made of a material having a refractive index n₃with respect to the light having the wavelength λ, and has a thickness Tsatisfying,when n₁>n₃ and n₂>n₃ , T≠(k×λ)/(2×n ₃), andwhen n₁>n₃>n₂ , T≠((2k−1)×λ)/(4×n ₃)where n₃≠n₁≠n₂, and k represents a natural number. This makes itpossible to determine the thickness uniformity of the waveguidesubstrate by the interference fringe observation.

Still further, the wavelength λ may be set to any one of 380 nm, 410 nm,441.6 nm, 488 nm, 532 nm, and 632.8 nm.

Still further, it is preferable that the waveguide substrate is made ofa transparent crystal, and the thin film layer has a multilayer filmstructure made of a plurality of dielectric materials having differentrefractive indices. The thin film layer is configured so that when thelight having the predetermined wavelength is incident perpendicularly onthe waveguide substrate, lights reflected at the respective layers havephase differences such that the reflected lights do not cancel oneanother. This makes it possible to determine the thickness uniformity ofthe waveguide substrate by the interference fringe observation. Theabove-mentioned “transparent crystal” may have a light transmissivitywith respect to light having a wavelength in a predetermined range.

Still further, the transparent crystal has a light transmissivity withrespect to light with a wavelength in a range covering any one of 380nm, 410 nm, 441.6 nm, 488 nm, 532 nm, and 632.8 nm.

Still further, another optical element of the present invention includesa waveguide substrate and a base substrate, in which the waveguidesubstrate and the base substrate are bonded with each other, and abonding surface at which the waveguide substrate and the base substrateare bonded includes, in a part thereof, a non-bonding region at whichthey are not bonded. This makes it possible to enhance the opticalwaveguide characteristics without providing a thin film layer,irrespective of the refractive indices of the waveguide substrate andthe base substrate.

Still further, it is preferable that the waveguide substrate and thebase substrate are bonded by direct bonding. This enables bonding withhigh precision.

Still further, it is preferable that the waveguide substrate and thebase substrate have refractive indices that are substantially equal toeach other. This enables bonding with high precision for the bondingbetween the waveguide substrate and the base substrate. Thisparticularly enables bonding with high precision in the case of thedirect bonding.

Still further, the waveguide substrate and the base substrate may bebonded with a thin film layer interposed therebetween.

Still further, it is preferable that at least one of bonding between thewaveguide substrate and the thin film layer and bonding between the basesubstrate and the thin film layer is direct bonding. This enablesbonding with high precision.

Still further, it is preferable that an optical waveguide is formed inthe waveguide substrate, the non-bonding region has a width not lessthan a width of a projection region defined by projecting the opticalwaveguide perpendicularly, and the projection region falls within thenon-bonding region. This makes it possible to achieve enhanced waveguidecharacteristics without waveguide losses.

Still further, the optical waveguide may be a ridge optical waveguide.

Still further, the non-bonding region may be provided by forming arecess in a thickness direction on at least one of the waveguidesubstrate and the base substrate.

Still further, it is preferable that the non-bonding region has athickness of not less than 0.01 μm. This makes it possible to achieveenhanced waveguide characteristics without waveguide losses.

Still further, it is preferable that a filler having a refractive indexsmaller than that of the waveguide substrate is provided in thenon-bonding region. This makes it possible to achieve enhanced waveguidecharacteristics without waveguide losses.

Still further, it is preferable that the filler is an ultravioletcurable resin. This facilitates the production, and makes it possible toachieve enhanced waveguide characteristics without waveguide losses.

Still further, the filler may be a dielectric material.

Still further, the filler may be provided in contact with the waveguidesubstrate and at least in a projection region defined by projecting theoptical waveguide perpendicularly.

Still further, it is preferable that a plurality of the non-bondingregions are provided, and are arranged at regular intervals. Thisincreases the strength at the bonding surface.

Still further, a method for producing an optical element of the presentinvention includes the steps of: forming a recess on a principal surfaceof at least one of a waveguide substrate and a base substrate, therecess being to function as a non-bonding region; and bonding thewaveguide substrate and the base substrate with each other so that thenon-bonding region is interposed between the waveguide substrate and thebase substrate. This makes it possible to produce an optical elementhaving excellent waveguide characteristics, irrespective of refractiveindices of the waveguide substrate and the base substrate.

Still further, it is preferable that in the recess forming step, therecess is formed on a principal surface of one of the waveguidesubstrate and the base substrate in a thickness direction. This makes itpossible to produce an optical element having excellent waveguidecharacteristics, irrespective of refractive indices of the waveguidesubstrate and the base substrate, without employing a thin film.

Still further, it is preferable that the recess forming step includes asub-step of forming a thin film on a principal surface of one of thewaveguide substrate and the base substrate, and forming the recess in athickness direction in the thin film. This makes it possible to producean optical element in which the thin film and the waveguide substrateare in contact with each other. Therefore, it is possible to produce anoptical element having excellent waveguide characteristics, irrespectiveof refractive indices of the waveguide substrate and the base substrate.

Still further, it is preferable that the bonding step includes sub-stepsof: forming a thin film on the principal surface having the recess;smoothening the principal surface having the thin film thereon by achemical mechanical polishing process; and bonding the waveguidesubstrate and the base substrate with each other by subjecting theprincipal surfaces thereof to direct bonding so that the non-bondingregion is interposed between the waveguide substrate and the basesubstrate. This makes it possible to produce an optical element in whichthe thin film and the waveguide substrate are in contact with eachother. Therefore, it is possible to produce an optical element havingexcellent waveguide characteristics, irrespective of refractive indicesof the waveguide substrate and the base substrate.

Still further, it is preferable that the bonding step includes: formingthe recess on the base substrate; forming a thin film on a principalsurface of the waveguide; and bonding the waveguide substrate and thebase substrate with each other so that the thin film falls within thenon-bonding region. With this, it is unnecessary to polish the thin filmlayer, and therefore, it is possible to reduce the number of steps.

Still further, it is preferable that the method further includes thestep of filling a filler having a refractive index smaller than arefractive index of the waveguide substrate in a gap formed by therecess between the waveguide substrate and the base substrate after thewaveguide substrate and the base substrate are bonded. This makes itpossible to produce an optical element in which the thin film and thewaveguide substrate are in contact with each other. Therefore, it ispossible to produce an optical element having excellent waveguidecharacteristics, irrespective of refractive indices of the waveguidesubstrate and the base substrate.

Still further, it is preferable that in the bonding step, the waveguidesubstrate and the base substrate are bonded by direct bonding. Thismakes it possible to produce an optical element with high bondingprecision.

Still further, another method for producing an optical element of thepresent invention includes the steps of: forming recesses in a thicknessdirection on a base substrate so as to form a plurality of groovesarranged at regular intervals; stacking one base substrate on another bybonding them so that groove-formed surfaces of the base substrates faceeach other and that the grooves of one of the base substrates cross thegrooves of the other base substrate, and polishing one of the basesubstrates until the grooves are exposed, so as to prepare a basesubstrate having a stacked structure; and repeating the stacking stepwith respect to the base substrate having a stacked structure. Thismakes it possible to produce a photonic crystal readily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a configuration of an opticalelement according to Embodiment 1 of the present invention.

FIGS. 2A and 2B are views showing measurement results of surfaceroughness of films formed by sputtering, which were measured by anatomic force microscope. FIG. 2A shows a measurement result of a surfaceroughness of a Ta oxide film formed by sputtering, and FIG. 2B shows ameasurement result of a surface roughness of a SiO₂ film formed bysputtering.

FIG. 3 is a perspective view illustrating another configuration of anoptical element according to Embodiment 1 of the present invention.

FIGS. 4A and 4B are orthographic drawings by the third angle projectionmethod of the optical element shown in FIG. 3.

FIG. 5 is a perspective view illustrating an optical element accordingto Embodiment 2 of the present invention.

FIG. 6 is a view for explaining reflected lights from respectivesurfaces of a thin film layer.

FIG. 7 is a graph showing a relationship of a light reflectance R withrespect to a thickness T of the thin film layer.

FIG. 8 is a perspective view illustrating a structure of an opticalelement according to Embodiment 3 of the present invention.

FIG. 9 is an orthographic drawing by the third angle projection methodillustrating how non-bonding regions of the optical element ofEmbodiment 3 are arranged.

FIG. 10 is a perspective view illustrating another configuration of anoptical element according to Embodiment 3 of the present invention.

FIG. 11 is a perspective view illustrating a configuration of an opticalelement according to Embodiment 4.

FIG. 12 is a perspective view illustrating another configuration of anoptical element according to Embodiment 4.

FIG. 13 is a perspective view illustrating still another configurationof an optical element according to Embodiment 4.

FIGS. 14A to 14D are front views of an optical element according toEmbodiment 5 of the present invention, which are shown according to anorder of a production process.

FIGS. 15A and 15B are a perspective view and a front view, respectively,illustrating a structure of an optical element according Embodiment 5 ofthe present invention.

FIG. 16 is a perspective view illustrating a configuration of an opticalelement according to Embodiment 6 of the present invention.

FIG. 17 is a perspective view illustrating another configuration of anoptical element according to Embodiment 6 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following will describe embodiments of the present invention. In thefollowing description, an optical-waveguide-type element is taken as anexample of an optical element. In the optical element, MgO-doped LiNbO₃crystal (hereinafter abbreviated as MgO:LN crystal), which is aferroelectric crystal, is used for forming a waveguide substrate inwhich an optical waveguide is formed, and LiNbO₃ crystal (hereinafterabbreviated as LN crystal) is used for forming a base substrate as abase of the optical element. However, the present invention is notlimited to this configuration.

EMBODIMENT 1

The direct bonding technique is known as a technique for firmly bondingsubstrates without using an adhesive or the like, and it allows variousmaterials such as glass, semiconductors, ferroelectrics, piezoelectricceramics, etc. to be bonded with high precision. A directly bondedsubstrate pair draws attention as an effective means for producing anoptical element, since one of the bonded substrates (the pair ofsubstrates bonded), formed in a thin plate form and caused to have aridge structure, can be used as an optical waveguide. Direct bondingprocesses, including both cases of direct bonding between substrates ofa same kind and direct bonding between substrates of different kinds,have been performed with respect to substrates of various kinds ofoxides such as LN crystal, LiTaO₃ crystal (hereinafter abbreviated as LTcrystal), MgO:LN crystal, and sapphire. Further, examples of the directbonding with a thin film interposed between substrates have been madepublic, in which a material such as SiO₂, SiN, low-melting glass, metaloxide, etc. is used for forming a thin film material.

An optical element according to Embodiment 1 is configured so that, forinstance, a ferroelectric crystal substrate made of LiNb_(x)Ta_((1-x))O₃(0≦x≦1) is bonded with another substrate with a thin film layer beinginterposed therebetween. As the foregoing thin film layer, particularlya thin film layer including a film made of Ta₂O₅ or Nb₂O₅ as a principalcomponent is used. It should be noted that the expression of “made ofTa₂O₅ or Nb₂O₅ as a principal component” means that Ta₂O₅ or Nb₂O₅ iscontained at a ratio of not less than 80%. Further, it is desirable thatTa₂O₅ or Nb₂O₅ is contained at a ratio of not less than 90%.

This configuration ensures high-precision control of a film thickness ofthe thin film layer. Therefore, it is possible to produce readily anoptical element in which, for instance, a substrate of MgO:LN crystal asan example of LiNb_(x)Ta_((1-x))O₃ crystal is bonded with a LN crystalsubstrate so that MgO:LN with excellent resistance to optical damagefunctions as an optical waveguide. It should be noted that aferroelectric crystal of LiNb_(x)Ta_((1-x))O₃ has optical nonlinearity,and the use of this crystal for forming an optical element makes itpossible to provide an optical modulator or a wavelength converterutilizing the nonlinear optical effect.

Effects of the use of the film containing Ta₂O₅ or Nb₂O₅ as a principalcomponent for forming the thin film layer were actually determined bythe inventers of the present invention, and they are shown below. First,since a thermal expansion coefficient of the thin film layer isapproximate to a thermal expansion coefficient of theLiNb_(x)Ta_((1-x))O₃ crystal substrate, the film does not exfoliate fromthe substrate even when subjected to high temperature. Further, a filmwith high smoothness can be achieved even by radio-frequency (RF)sputtering, which is a relatively simple film forming technique.Further, in a direct bonding process including ahydrophilicity-imparting treatment and a subsequent process formanufacturing an optical element, the film exhibits high chemicalresistance and minimum deterioration during the treatment. Stillfurther, the film has an excellent strength, and an excellentadhesiveness to a LiNb_(x)Ta_((1-x))O₃ crystal substrate. Therefore,direct bonding with high strength can be achieved.

Among metal oxides, Ta₂O₂ and Nb₂O₅ are particularly suitable asmaterials for forming a thin film layer for use in the direct bonding ofa ferroelectric crystal of LiNb_(x)Ta_((1-x))O₃. Further, a Ta₂O₅ filmhas a refractive index lower than that of a Nb₂O₅ film, and lower thanthat of the LiNb_(x)Ta_((1-x))O₃ crystal substrate. Therefore, a Ta₂O₅film is particularly suitable for producing an optical-waveguide-typeelement in which a LiNb_(x)Ta_((1-x))O₃ crystal substrate is used.

One of the advantageous effects of the direct bonding technique is thatit allows substrates having different properties to be bonded firmlywith each other with high precision, thereby making it possible toprovide devices having various characteristics, and hence, the directbonding of substrates of different kinds has a significant advantage.The simplest examples of the direct bonding of substrates of differentkinds using ferroelectric crystals include, for instance, the directbonding of glass with LN crystal or the direct bonding of glass with LTcrystal. However, considering that the heat treatment at a temperaturein a range of several hundred ° C. to approximately 1000° C. is carriedout as a general direct bonding process, it is required that thesubstrates to be bonded have substantially equal thermal expansioncoefficients. Therefore, a method of using materials havingsubstantially equal thermal expansion coefficients and differentrefractive indices, such as LN crystal and MgO:LN crystal, is consideredeasier, as compared with the foregoing configuration (the direct bondingof glass with LN crystal, the direct bonding of glass with LT crystal).

The direct bonding of LN crystal with MgO:LN crystal has been proposed,and optical elements of an optical waveguide type formed using the samehave been disclosed. In this case, LN crystal is used as an opticalwaveguide since it has a refractive index greater than that of MgO:LNcrystal. Both of LN crystal and MgO:LN crystal are ferroelectriccrystals having optical nonlinearity, and have substantially equalnonlinear optical constants. Further, a LN crystal optical waveguideoften is used in the case of an optical modulator utilizing thenonlinear optical effect or in the case where wavelength conversion isperformed with respect to long wavelengths, since LN crystal isinexpensive as compared with MgO:LN crystal. However, it is difficult touse a LN crystal optical waveguide in an optical-waveguide-typewavelength converter that converts a wavelength to a short wavelength ofnot more than 500 nm, since a local non-uniform change in the refractiveindex (the phenomenon generally referred to as optical damage) occursupon irradiation of light.

On the other hand, MgO:LN crystal is known as a material with anexcellent resistance to optical damage, and particularly is regarded asa promising material for use in a wavelength converter for conversion toshort wavelengths. Therefore, in this case, MgO:LN crystal is used asoptical waveguide preferably.

Thus, by the method, it is possible to form an optical elementirrespective of refractive indices of respective substrates, from thestandpoint of function and cost performance of the direct bondsubstrate.

It has been proposed to arrange an intermediate layer (thin film layer)made of SiO₂, SiN, low-melting glass, etc. between substrates, regardingan optical element to which the direct bonding is applied. For instance,JP06-289347A discloses a device manufactured by direct bonding, in whichan intermediate layer made of a material having a refractive index lowerthan that of a substrate material for forming an optical waveguide, anda method for manufacturing the same. This makes it possible to useMgO:LN crystal having a refractive index lower than that of LN crystalas an optical waveguide. In the case where low-melting glass is used forinstance, however, a technique such that a glass material dissolved in athinner or the like is applied over substrates to be bonded andthereafter the substrates are brought into close contact with eachother, subjected to pressure and baked is used as a technique of directbonding of low-melting glass with an intermediate layer. Therefore, aproblem arises in that the control of the thickness uniformity of thelow-melting glass is difficult. Further, a SiO₂ film or a SiN filmformed by a simple method such as vapor deposition or sputtering cannotbe subjected to direct bonding since it has an inferior surfacesmoothness.

Therefore, the inventors of the present invention focused on adielectric thin film that allows for the thickness uniformity control,as a layer to be formed between substrates bonded directly, and examinedcharacteristics thereof. As a result, they found that dielectric filmscontaining Ta₂O₅ and Nb₂O₅ as principal components, for instance, areapplicable.

The following will describe an optical element according to Embodiment 1of the present invention, while referring to the drawings. FIG. 1 is aperspective view illustrating a configuration of the optical elementaccording to Embodiment 1. As shown in FIG. 1, the optical elementaccording to Embodiment 1 has a configuration in which a base substrate2 and a waveguide substrate 1 are stacked with a thin film layer 4interposed therebetween.

For instance, an X-cut MgO:LN crystal substrate is used as the waveguidesubstrate 1. An X-cut LN crystal substrate is used as the base substrate2. Further, the waveguide substrate 1 is processed so as to have a ridgestructure, whereby a ridge-type optical waveguide 3 is formed. The thinfilm layer 4 is a dielectric single-layer film containing Ta₂O₅ as aprincipal component.

A method for producing an optical element according to Embodiment 1 isdescribed. First, the thin film layer 4 is formed on a principal surfaceof either the waveguide substrate 1 or the base substrate 2. The RFsputtering, for instance, may be used for forming the thin film layer 4,so that a Ta₂O₅ thin film as the thin film layer 4 is formed with athickness of 100 nm to 400 nm on, for instance, the waveguide substrate1 of MgO:LN crystal. Here, the refractive index of the thin film layer 4containing Ta₂O₅ as a principal component varies with sputteringconditions. As actual measurement results, the refractive index variedin a range of 2.05 to 2.10 with respect to light having a wavelength of623.8 nm. Since MgO:LN crystal as the waveguide substrate 1 has arefractive index of approximately 2.17 with respect to the light withthe same wavelength, it is possible to cause the optical elementaccording to Embodiment 1, when finished, to trap and guide light with awavelength of 623.8 nm through the optical waveguide 3.

After the thin film layer 4 is formed on the waveguide substrate 1, asurface of the thin film layer 4 and a surface of the base substrate 2of LN crystal, which are surfaces to be subjected to direct bonding, aresubjected to a treatment for imparting hydrophilicity. Morespecifically, after the thin film layer 4 formed on the waveguidesubstrate 1 and the base substrate 2 are subjected to acetone ultrasoniccleaning, they are immersed in a mixture solution of ammonia water(aqueous ammonia), hydrogen peroxide, and deionized water at a ratio of1:1:6 (hereinafter referred to as ammonia-hydrogen peroxide solution)for not less than fifteen minutes, rinsed with deionized water, anddried. It should be noted that normal ammonia water having aconcentration of 20% to 40% may be used. Desirably, the ammonia waterhas a concentration of 30%.

Thereafter, the waveguide substrate 1 and the base substrate 2 arecombined with each other so that the directions of the respectivecrystallographic axes of the waveguide substrate 1 and the basesubstrate 2 coincide with each other and that the surface of the thinfilm layer 4 and the surface of the base substrate 2 are brought intocontact, and the substrates are subjected to a slight pressure. By sodoing, the thin film layer 4 and the base substrate 2 are brought intoclose contact with each other. Further, by subjecting the base substrate2 and the waveguide substrate 1 having the thin film layer 4 thereon toa heat treatment, the thin film layer 4 and the base substrate 2 arebonded directly. The heat treatment is carried out at 350° C. to 800°C., with use of an oven, at a temperature rising rate of, for instance,100° C./h to 500° C./h for preventing the delamination of the bondedsubstrates and damages due to rapid temperature rise.

It should be noted that the direct bonding is a technique for firmlybonding two substrates without using an adhesive, a resin, or the like.For instance, in the foregoing method, the substrates are brought intoclose contact with each other by hydrogen bonds of OH groups. Therefore,the substrates are brought into a further firm bonding state by afurther heat treatment, since the OH groups form H₂O molecules and leavethe bonding surface (dehydration). Further, apart from this method,there is another direct bonding method utilizing electrostaticattraction.

To form the optical waveguide 3 in the paired substrates thus formed bydirect bonding (hereinafter sometimes referred to as a direct-bondsubstrate), the waveguide substrate 1 is subjected to surface polishingand thinning, so as to finally have a thickness of 3 μm to 4 μm.Thereafter, an optical waveguide patterning is carried out byphotolithography, and a ridge of 1.5 μm to 2 μm is formed by dry-etchingon the surface of the waveguide substrate 1 thus thinned. Thus, theridge-type optical waveguide 3 is formed. It should be noted that bothend faces (entrance and outgoing faces) of the optical waveguide 3 aresubjected to mirror-finishing after the ridge is formed.

Here, the thin film layer 4 is described in more detail. Generally, itis known that in the case where a dielectric film is formed on asubstrate by sputtering, vapor deposition, etc., characteristics of thefilm thus formed vary depending on film forming conditions (temperature,ambient atmosphere for the film formation, etc.). Besides, it is knownthat a surface state (surface roughness) of a film significantly variesdepending on a film forming method including a film forming device. Forinstance, depending on the technique applied, such as vapor deposition,RF sputtering, electron-cycrotron-resonance (ECR) sputtering, or CVD,and a device used, the film formed has different homogeneity, density,and surface roughness. Still further, it is known that even when thesame method is used, the use of a different material results in adifferent surface state.

However, in the optical element according to Embodiment 1, when the thinfilm layer 4 and the base substrate 2 are bonded, the surface roughnessof the thin film layer 4 formed on the waveguide substrate 1 affects thebonded state. For instance, in the case where the surface issignificantly rough, the optical element has deteriorated precision. Inthe case where the thin film layer 4 and the base substrate 2 are to bebonded directly in particular, an excessive roughness of the surface ofthe thin film layer 4 makes the direct bonding impossible. For instance,it is known that direct bonding cannot be used with respect to a SiO₂film formed by RF sputtering or ECR sputtering since it has asignificantly rough surface. In light of these experiment results,generally the direct bonding employing a dielectric film formed bysputtering is not carried out. However, as in the present invention, inthe case where the thin film layer 4 is a single-layer or multi-layerthin film containing Ta₂O₅ or Nb₂O₅ as a principal component, the thinfilm layer 4 has a surface with an extremely small roughness even if itis formed by sputtering, and hence, it has a roughness precisionrequired for the direct bonding.

FIGS. 2A and 2B are views showing roughnesses of surfaces of filmsformed by sputtering. FIG. 2A shows a measured result of a surfaceroughness of a Ta oxide film formed by sputtering (target as sputteringsource is Ta₂O₅), and FIG. 2B shows a measured result of a surfaceroughness of a SiO₂ film formed by sputtering, both of which weremeasured by an atomic force microscope. As seen in FIGS. 2A and 2B, thesurface roughness (maximum: approximately 2 nm) of the Ta oxide isconsiderably smaller than the surface roughness of SiO₂ (maximum:approximately 6 nm). A surface roughness at an equal level of that ofthe Ta oxide film formed by sputtering can be obtained with, forinstance, a niobium oxide film formed by sputtering (target assputtering source is Nb₂O₅), but the niobium oxide film has a refractiveindex of 2.25 to 2.35 with respect to light having a wavelength of 632.8nm, which is greater than that of MgO:LN crystal. Therefore, in the casewhere the waveguide substrate 1 is MgO:LN crystal, light cannot betrapped, and the optical waveguide 3 cannot be formed. However, in thecase where a material having a further higher refractive index, forinstance, a semiconductor material such as a silicon wafer, is used forforming the waveguide substrate 1, it is possible to use a niobium oxidefor forming the thin film layer 4. The thin film layer 4 is made to havea refractive index lower than that of the waveguide substrate 1, and asuitable one may be selected from a dielectric film containing Ta₂O₅ asa principal component and a dielectric film containing Nb₂O₅ as aprincipal component, depending on a material used for forming thewaveguide substrate 1.

Furthermore, a dielectric film containing Ta₂O₅ or Nb₂O₅ as a principalcomponent has been proved experimentally to have a high chemicalresistance. This is a necessary property for the direct bonding. Forinstance, the above-described ammonia-hydrogen peroxide treatment forimparting hydrophilicity is carried out for the direct bonding, and theSiO₅ film formed by sputtering is not suitable as the thin film layer 4for direct bonding since the ammonia-hydrogen peroxide solution erodesthe film and further deteriorates the in-plane uniformity of the filmthickness. On the other hand, a dielectric film containing Ta₂O₅ orNb₂O₅ as a principal component is not eroded by the ammonia-hydrogenperoxide solution, and has a strong resistance against an organicsolvent or a buffered hydrogen fluoride (a mixture solution of hydrogenfluoride:ammonium fluoride:water=1:5:50), which are used in otherprocesses.

Here, a result obtained regarding an optical element according toEmbodiment 1 is shown below, which was produced actually by using, asthe thin film layer 4, a dielectric film containing Ta₂O₅ as a principalcomponent. A pair of substrates directly bonded with the thin film layer4 containing Ta₂O₅ as a principal component being interposedtherebetween did not exhibit delamination or deterioration in a processsubsequent to the direct bonding (the thinning and polishing,photolithography, dry-etching, and end face polishing process withrespect to the waveguide substrate 1 made of MgO:LN crystal), and anextremely excellent bonded state was maintained. Particularly, thestrength of the Ta₂O₅ film formed by sputtering as the thin film layer 4and the adhesion thereof to the waveguide substrate 1 were reinforced bythe film formation in an atmosphere at a temperature of not lower than100° C. as the sputtering condition, and an effect was achieved inpreventing the delamination or deterioration of the bonding surface andthe sputtered surface upon the thinning and polishing process.

Further, waveguide characteristics of the optical waveguide 3 formedwere excellent, and a sufficient light trapping effect and waveguidingwith low losses were observed. Furthermore, in order to form asingle-mode optical waveguide 3, it is necessary to control therefractive index of the thin film layer 4 and to control the shape ofthe optical waveguide 3 precisely. The use of a material having arefractive index approximate to that of MgO:LN crystal of the waveguidesubstrate 1, as a dielectric material containing Ta₂O₅ as a principalcomponent, makes it possible to achieve an effect of significantlyincreasing the design tolerance for designing the optical waveguide.

Not only in the case where a dielectric film containing Ta₂O₅ as aprincipal component is used as the thin film layer 4, but also in thecase where a dielectric film, containing Nb₂O₅ as a principal componentis used as the thin film layer 4, an extremely excellent bonded state ofdirect bonding was maintained. Further, the waveguiding of light throughthe optical waveguide 3 was excellent. It should be noted that Nb₂O₅ hasa refractive index higher than that of Ta₂O₅, and a suitable materialmay be selected from these materials according to the material used forforming the waveguide substrate 1.

So far the case where a dielectric single-layer film containing Ta₂O₅ asa principal component is used as the thin film layer 4 has beendescribed, but a multi-layer film containing Ta₂O₅ as a principalcomponent may be used as the thin film layer 4. Films with differentrefractive indices can be formed by varying a film forming atmospherefor forming the dielectric film (flow rates of Ar gas and O₂ gas, apressure inside a chamber of a sputtering device, etc.), or varying afilm forming temperature or a voltage that the sputtering deviceapplies. For instance, on the waveguide substrate 1 made of MgO:LNcrystal (having a refractive index of 2.17 with respect to light havinga wavelength of 632.8 nm), thin films containing, as their principalcomponents, two kinds of Ta₂O₅ which have refractive indices of 2.05 and2.10, respectively, with respect to light having a wavelength of 632.8nm may be formed so as to compose a multi-layer film. The thin films incontact with the waveguide substrate 1 and the base substrate 2 are madeto have a refractive index of 2.05. A thin film of one of the kinds thathas a refractive index of 2.05 and a thin film of the other kind thathas a refractive index of 2.10 may be formed alternately so that theformer has a thickness of 77 nm and the latter has a thickness of 75 nm.A multi-layer film composed of nine layers thus formed as the thin filmlayer 4 has a reflectance of 2.3%, while a multi-layer composed ofnineteen layers as the thin film layer 4 has a reflectance of 7.1%. Inother words, as the layers increase in number, the reflectanceincreases. On the other hand, a single-layer film containing Ta₂O₅ as aprincipal component has a reflectance of 0.32%. Thus, a reflectance ofnot less than ten times the reflectance of the single-layer film can beobtained easily.

Likewise, a multi-layer film made of a niobium oxide, or a multi-layerfilm made of a Ta oxide or a niobium oxide, can be subjected to directbonding as the thin film layer 4. By so doing an optical waveguide-typeelement suffering from low losses and having a sufficient light trappingeffect can be produced. In the case where a multi-layer film made of aTa oxide and a niobium oxide is used, for instance, the film may have aconfiguration in which a Ta oxide film layer having a lower refractiveindex is formed so as to be in contact with the optical waveguide 3, andthe thin film layer 4 is formed using a niobium oxide film layer havinga higher refractive index on the other side, so that the base substrate2 and the niobium oxide film layer are bonded directly. With thisconfiguration, it is possible to control the light-trapping state sothat a desirable state thereof can be achieved.

Further, the thin film layer 4 may have a multi-layer structureincluding a film containing Ta₂O₅ or Nb₂O₅ as a principal component anda metal layer made of a metal material. For instance, the thin filmlayer 4 may have a metal layer on its surface to be subjected to directbonding.

Reflected light from a rear face of the direct-bond substrate appears asnoise components with respect to interference fringes formed byreflected light from a front face of the waveguide substrate 1 andreflected light from the thin film layer 4. For instance, in the casewhere dust is mixed in or an adhesive is applied unevenly when thewaveguide substrate 1 is attached to a holding table that is used in thepolishing work, interference fringes sometimes are observed that arecaused by reflected light from a rear face of the direct-bond substrate(a face of the direct-bond substrate in contact with the holding table)and reflected light from a surface of the waveguide substrate 1 sincethe rear face of the direct-bond substrate is tilted with respect to thesurface to be polished. These interference fringes hinder theobservation of interference fringes due to reflected light from the thinfilm layer 4 and reflected light from a surface of the substratethinned, which should be observed. To cope with this problem, a metallayer is arranged as an intermediate layer for direct bonding, so thatsuch noise components are removed, while the thinning of the substrateto be thinned can be facilitated.

As shown in FIG. 3, the thin film layer 4 is formed on the basesubstrate 2, and the waveguide substrate 1 having the ridge-type opticalwaveguide 3 is formed thereon. The thin film layer 4 is composed of ametal layer 5 made of a metal material such as Ta and a dielectric layer6 containing Ta₂O₅ or Nb₂O₅ as a principal component. For instance, themetal layer 5 is formed on the base substrate 2 side, while thedielectric layer 6 is formed on the waveguide substrate 1 side. Itshould be noted that the optical element is manufactured by forming thethin film layer 4 on the base substrate 2 that is an X-cut LN crystalsubstrate, thereafter, bonding the waveguide substrate 1 that is anX-cut MgO:LN crystal substrate and the thin film layer 4 with each otherby direct bonding, and forming the optical waveguide 3 in the waveguidesubstrate 1. A Ta film, for instance, is used as the metal layer 5 ofthe thin film layer 4, while a single-layer film containing Ta₂O₅ as aprincipal component is used as the dielectric layer 6. It should benoted that any metal may be used for forming the metal layer 5, as longas the same effect as that described above can be achieved.

Strong bonding by direct bonding cannot be achieved at an interfacebetween the metal layer 5 and the waveguide substrate 1 made of MgO:LNcrystal that is a ferroelectric crystal, or an interface between themetal layer 5 and the base substrate 2 made of LN crystal. On the otherhand, the dielectric layer 6 composed of a single-layer film containingTa₂O₅ as a principal component and the waveguide substrate 1 can bebonded excellently by direct bonding. Therefore, they are bondeddirectly. Here, if the metal layer 5 is excessively close to the opticalwaveguide 3, this causes absorption losses of guided light. Forinstance, with an excessive power of guided light, damage to the opticalelement possibly occurs. Therefore, it is necessary to separate themetal layer 5 from the optical waveguide 3 with at least a distancecorresponding to the evanescent range of guided light. A distance of notless than 50 nm is sufficient between the metal layer 5 and thewaveguide substrate 1. It should be noted that the evanescent range is arange in which light propagating through an optical waveguide leaks out.If the metal layer 5 is arranged in this range, guided light possibly isaffected by the metal layer 5 and causes damage to the optical element.

In the manufacture of an optical element by direct bonding, first, themetal layer 5 and the dielectric layer 6 are formed successively by RFsputtering on the base substrate 2. The metal layer 5 and the dielectriclayer 6 are formed to as to have a thickness of 5 nm to 100 nm, and athickness of 10 nm to 200 nm, respectively.

Thereafter, the thin film layer 4 and the waveguide substrate 1 arebonded directly as described above, and the optical waveguide 3 isformed. In this optical element, a distance between the metal layer 5and the optical waveguide 3 (waveguide substrate 1) is equal to athickness of the dielectric layer 6. In the case where this thicknesswas not less than 50 nm, the optical element had excellent waveguidecharacteristics, and a sufficient light trapping effect and thewaveguiding with low losses were observed.

FIGS. 4A and 4B are orthographic drawings by the third angle projectionmethod of the optical element shown in FIG. 3. As shown in a plan viewof FIG. 4A, the metal layer 5 is not formed over an entirety of thesurface of the bond substrate, but formed by patterning so thatmetal-removed portions 5 a where the metal layer 5 is not formed arepresent. It should be noted that the metal layer 5 actually is coveredwith the thin film layer 6 and the waveguide substrate 1 laminatedthereon, but FIG. 4A shows the metal layer 5 as if it were seen throughthe same.

By patterning the metal layer 5 as shown in FIG. 4A, the positioning ofthe optical waveguide 3 when it is formed is facilitated. Morespecifically, the metal-removed portions 5 a are utilized as markerswhen the positioning is carried out. This enables the highly precisemounting of the optical element. The optical waveguide 3 of the opticalelement made of LN crystal or MgO:LN crystal that are used for formingthe waveguide substrate 1 and the base substrate 2 have waveguide lossesdependent on the crystallographic axis directions thereof. For instance,in the case where the optical waveguide 3 for transmission in the Y-axisdirection is formed using a X-cut substrate of LN crystal, the waveguidelosses increase greatly if the waveguide is formed in a direction tiltedat an angle of several degrees from the Y axis. Accordingly, it isimportant to direct the optical waveguide 3 in a direction such thatdesired characteristics are obtained, with the crystallographic axisdirection taken into consideration.

Furthermore, in the case where a wavelength converter is produced usingthe optical waveguide 3 in particular, it is necessary to form aperiodic polarization reversal structure in which the polarizationdirection of crystal is inverted cyclically so as to increase awavelength conversion efficiency. It is known that LN crystal polarizedto have a single polarity has a polarization direction in a C-axisdirection (=Z-axis direction) of the crystal, and by applying ahigh-voltage electric field using periodic electrodes, for instance, theforegoing cyclic polarity inversion can be achieved. For the wavelengthconversion, the oscillation direction and polarization direction of alight wave preferably coincide with each other. Therefore, to performthe high-efficient wavelength conversion, the cyclic polarity inversionstructure and the optical waveguide 3 preferably are formed so as tocross orthogonally. Thus, in the manufacture of an optical element inwhich a direct-bond substrate is used, for instance a wavelengthconverter, it is necessary to specify the crystallographic axisdirection.

The conventional optical waveguide manufactures did not have a techniquefor specifying a direction of a crystal substrate. Therefore, it wasnecessary to form positioning-use markers, for instance, by lasermarking on a rear face of a direct-bond substrate (a face opposite tothe optical waveguide-formed face). However, by bonding the waveguidesubstrate 1 and the base substrate 2 with the thin film layer 4including the patterned metal layer 5 being interposed therebetween, thepositioning-use markers are unnecessary. More specifically, the thinfilm layer 4 including the patterned metal layer 5 is formed on the basesubstrate 2, and when the waveguide substrate 1 is arranged thereon, thebonding is performed by referring to the metal-removed portions 5 aformed by the patterning. By so doing, the crystallographic axisdirections are made to coincide with each other.

Furthermore, the metal layer 5, formed in the vicinity of alight-incident end 3 a of the optical waveguide 3, tends to cause damageto the element due to light absorption with respect to light when lightfrom a laser is coupled with the optical waveguide 3. Therefore, asshown in FIG. 4B, it is effective to provide a metal removed portion 5 bin the vicinity of the light-incident end 3 b of the optical waveguide3. According to actual measurement, the absence of a metal layer atleast in a range of 1 μm from a surface of the light-incident end 3 a ofthe optical waveguide 3 was sufficient. In other words, themetal-removed portion 5 b preferably has a depth d of not less than 1μm.

Embodiment 1 is described by referring to a case where an opticalelement is produced by directly bonding a ferroelectric crystalsubstrate made of LiNb_(x)Ta_((1-x))O₃ with another crystal substrate,with the thin film layer 4 being interposed between the substrates, thethin film layer 4 being a dielectric film containing Ta₂O₅ or Nb₂O₅ as aprincipal component. However, even in the case where a substrate otherthan the LiNb_(x)Ta_((1-x))O₃ substrate is used, for instance, in thecase where the direct bonding is performed with respect to a Si wafer ora SiO₂ substrate with the interposition of the thin film layer, the useof the thin film layer 4 that is a dielectric film containing Ta₂O₅ orNb₂O₅ as a principal component is effective, since it has highsmoothness and high chemical resistance.

EMBODIMENT 2

An optical element according to Embodiment 2 is configured so as toallow highly precise evaluation of substrate thickness uniformity to beperformed when one of the substrates to be bonded with a thin filminterposed therebetween is thinned. The following will describe indetail what enables the thinning of the substrate for achieving a highthickness uniformity. It should be noted that Embodiment 2 is describedreferring to an example of the manufacture of an optical element inwhich a thinned substrate is used. The optical element of the presentembodiment is formed by directly bonding substrates made of LN crystaland MgO:LN crystal, in which the substrate made of MGO:LN crystal isthinned and an optical waveguide is formed therein. However, the presentinvention is not limited to an optical element of this configuration andof an optical waveguide type, and the bonding method is not limited tothe direct bonding.

Many techniques for forming an optical waveguide in LN crystal or MgO:LNcrystal have been proposed. For instance, as a method for forming anoptical waveguide in a wavelength converter utilizing non-linear opticalcharacteristics of LN crystal or MgO:LN crystal, the proton exchangemethod and the titanium diffusion method have been proposed. However, itis known that these methods entail deterioration of non-linear opticalconstants since they utilize refractive index variation caused byimplantation of impurities in the crystal. On the other hand, an opticalwaveguide-type element employing bonded substrates has an advantage ofnot deteriorating the characteristics of the crystal in principle, sinceit has a light trapping effect in the substrate thickness direction byutilizing a difference between refractive indices of the substratesbonded, and an optical waveguide is formed only by changing a shape ofthe direct-bond substrate. Therefore, this technique can be consideredsignificantly effective as a technique for producing an opticalwaveguide device that utilizes the non-linear optical effect asdescribed above.

Generally, the optical waveguide-type element formation is required toachieve high uniformity of an optical waveguide shape (thickness andwidth). In an optical waveguide-type wavelength converter having acyclic polarity inversion structure particularly, wavelength conversionof high efficiency is achieved in the case where phase matching betweeninput fundamental and harmonic is achieved uniformly throughout theoptical waveguide. Since the phase matching wavelength is determinedaccording to a polarity inversion period and effective refractiveindices of fundamental and harmonic, the conversion efficiencysignificantly decreases when the optical waveguide shape (width andheight) varies with respect to a waveguiding direction. The opticalwaveguide width depends on the precision of patterning of thephotoresist that is used as an etching mask, for instance, in the casewhere a ridge-type optical waveguide is formed by dry-etching.Therefore, high precision of a submicronic level is enabled.

On the other hand, the control of the height of the optical waveguide isachieved by the thinning of a substrate by polishing, for instance,where, however, the control of the height at a submicronic level isdifficult. The reason is that simple methods for measuring an absolutevalue of a height and uniformity of the same are limited, and theavailable methods are the thickness absolute value measuring methodusing a level difference meter and the uniformity evaluating method ofprojecting light to the substrate and measuring reflected lights from afront face and a rear face of a direct-bond substrate by aninterferometer. To form an optical waveguide in MgO:LN crystal indirectly bonded substrates made of MgO:LN crystal and LN crystal,respectively, as shown in Embodiment 1 in particular, the direct bondingutilizing a thin film layer interposed is effective. However, with athin film layer that has been proposed, sufficient reflected lightcannot be obtained from a thin film layer on a rear face of a substrateto be thinned, and the uniformity evaluation with use of aninterferometer cannot be carried out. Therefore, it is difficult toachieve highly precise uniformity.

The following will describe an optical element according to Embodiment2, while referring to the drawings. Since an optical waveguide is formedby substrates directly bonded with a thin film interposed therebetweenas shown in Embodiment 1, a substrate to be thinned, among thesubstrates to be bonded, desirably has a thickness with highly preciseuniformity. The optical element according to Embodiment 2 is an opticalelement that enables highly precise evaluation of a thickness of asubstrate where an optical waveguide is formed, and that thereforeenables the thinning with high thickness uniformity. It should be notedthat Embodiment 2 is described referring to an optical element employinga thinned substrate. In the optical element, LN crystal is used forforming a base substrate, MgO:LN crystal is used for forming a waveguidesubstrate, these base substrate and waveguide substrate are bonded witha thin film layer interposed therebetween, the waveguide substrate isthinned, and an optical waveguide is formed. However, the materials andconfiguration of the optical element are not limited to these describedherein.

In an optical element according to Embodiment 2 configured by bondingsubstrates with a thin film layer interposed therebetween, a waveguidesubstrate to be thinned is transparent with respect to light having aspecific wavelength λ and has a refractive index n₁ with respect to thespecific light with a wavelength λ. A base substrate bonded with thewaveguide substrate via the thin film layer has a refractive index n₂with respect to the light having a wavelength λ. Further, the thin filmlayer used as an intermediate layer between the waveguide substrate andthe base substrate has a refractive index n₃ (≠n₁≠n₂) with respect tothe light having a wavelength λ and a thickness T satisfying therelationship T≠(k×λ)/(2×n₃) where k represents a natural number. Thesatisfaction of the foregoing conditions makes it possible to determineuniformity of the waveguide substrate by utilizing a highly preciseuniformity of the thickness of the thin film layer and reflected lightfrom the thin film layer, and to perform the thinning of the waveguidesubstrate. More specifically, interference fringes caused by reflectedlight from a surface of the waveguide substrate to be thinned andreflected light from the thin film layer are observed, whereby thethickness uniformity determination of the crystal substrate is enabled.By so doing, an optical element having a thinned substrate with a highuniformity is provided. An optical waveguide-type element is produced bymaking the thinned substrate have a ridge structure.

FIG. 5 is a perspective view illustrating a structure of an opticalelement according to Embodiment 2 of the present invention. An exampleof an optical element is shown, which is formed by bonding a MgO:LNcrystal substrate and a LN crystal substrate with a thin film layerinterposed therebetween. In FIG. 5, an X-cut MgO:LN crystal substrate asa base substrate 2 and an X-cut LN crystal substrate as a waveguidesubstrate 1 are bonded with each other with a thin film layer 4interposed therebetween. It should be noted that the waveguide substrate1 and the thin film layer 4 are bonded by direct bonding. Morespecifically, the thin film layer 4 is formed on the base substrate 2 bysputtering or the like, and the thin film layer 4 that is a dielectricsingle-layer film containing Ta₂O₅ as a principal component is bondeddirectly with the waveguide substrate 1 that is the X-cut LN crystalsubstrate. It should be noted that a state like this in which thewaveguide substrate 1 and the base substrate 2 are bonded with the thinfilm layer 4 interposed therebetween is referred to as a direct-bondsubstrate. Thereafter, the waveguide substrate 1 of the direct-bondsubstrate is thinned, whereby an optical element is produced. It shouldbe noted that the thin film layer 4 is a dielectric single-layer filmcontaining Ta₂O₅ as a principal component.

In the case where the waveguide substrate 1 of the direct-bond substrateis thinned by polishing, it is necessary to make the thickness of thethinned waveguide substrate uniform. To achieve this, it is required tomaintain several factors highly precisely, for instance, apart from theparallelism (uniformity of thickness) of the direct-bond substrate, thesurface smoothness precision of a holding table for holding thedirect-bond substrate so that it does not move when being polished, theadhesion uniformity of the direct-bond substrate with the holding table,the film thickness uniformity of the thin film layer 4, etc.

To ensure that the waveguide substrate 1 after being thinned has lessthickness non-uniformity, a waveguide substrate 1 and a base substrate 2that have high degrees of parallelism should be used. Further, uponpolishing, the parallelism of the holding table on which the direct-bondsubstrate is caused to adhere is secured. Still further, since thedirect-bond substrate is polished in a state of adhering to the holdingtable with an adhesive, the non-uniformity of thickness of the adhesiveunavoidably results in the non-uniformity of thickness of the thinnedwaveguide substrate. To prevent this, a thermosetting adhesive, forinstance, is applied uniformly over a rear face of the direct-bondsubstrate by spin-coating or the like, and the direct-bond substrate ismade to adhere to the holding table by applying pressure and heatthereto. However, even with this process, distortion of the direct-bondsubstrate due to this adhesion occurs though it is slight. Therefore,even if the thinning is carried out by referring to the rear face of thedirect-bond substrate (the surface at which the holding table and thedirect-bond substrate are brought into contact) as a reference,non-uniformity of the thickness occurs. Therefore, the following methodis used further.

Generally, in the case where the waveguide substrate 1 is transparent,the interference fringe observing method, which is a simple and highlyprecise optical technique, is available for determining thicknessuniformity. The method of observing interference fringes is a method fordetermining a thickness uniformity of a substrate by projecting, forinstance, a laser with a wavelength of 633 μm to a surface of asubstrate and observing a state of interference between light reflectedfrom the substrate surface and reflected light from a surface of a thinfilm layer beneath the substrate surface. In the case where thethickness varies, interference fringes occur. Thus, the thicknessnon-uniformity can be determined easily. However, in the case of thethinning of the waveguide substrate 1 of the direct-bond substrate withthe thin film layer 4, it is difficult to observe interference fringessince the reflected light from the thin film layer 4 is weak.

Therefore, the optical element according to Embodiment 2 has aconfiguration such that the reflected light from the thin film layer 4has a sufficient intensity. FIG. 6 explains reflected lights onrespective surfaces of the thin film layer 4. FIG. 7 illustrates therelationship of a light reflectance R with respect to a thickness T ofthe thin film layer 4. In FIG. 6, reflected light 8 is light reflectedat an interface between the thin film layer 4 and the waveguidesubstrate 1, and reflected light 9 is light reflected at an interfacebetween the thin film layer 4 and the base substrate 2. Generally, areflectance R of Fresnel reflection at a medium interface between twomedia having refractive indices of n_(a) and n_(b), with respect tolight that is incident perpendicularly from the medium with n_(a) to themedium with n_(b) is expressed as:R=|(n _(a) −n _(b))/(n _(a) +n _(b))|²×100(%)

Calculated values are shown below in the case where, for instance, thewaveguide substrate 1 is a MgO:LN crystal substrate, the base substrate2 is a LN crystal substrate, and the thin layer 4 is Ta₂O₅. Forinstance, assuming that the waveguide substrate 1 has a refractive indexof 2.166, the thin film layer 4 has a refractive index of 2.10, and thebase substrate has a refractive index of 2.23 with respect to light witha wavelength of 632.8 nm, the reflectance R is calculated by theforegoing formula. The result of the calculation proves thatreflectances R are very small, with a reflectance R at an interfacebetween the waveguide substrate 1 and the thin film layer 4 being0.024%, and a reflectance R at an interface between the base substrate 2and the thin film layer 4 being 0.09%. Here, if the reflected light istoo weak, the observation of interference fringes by reflected lights isimpossible. Regarding this, the thickness T may be set so as to satisfythe following condition:T≠(k×λ)/(2×n ₃)where λ represents a wavelength of light, n₃ represents the refractiveindex of the thin film layer 4, and k represents a natural number. By sodoing, the reflected light 8 and the reflected light 9 interfere witheach other, and the reflected light from the thin film layer 4 isincreased consequently.

In the case where, for instance, light from a light source used for theinterference fringe observation has a wavelength λ of 632.8 nm and thethin film layer 4 (Ta₂O₅) has a refractive index n₃ of 2.1 with respectto the wavelength λ, as clear from FIG. 7, the thin film layer 4 havinga thickness T of 150.67 nm or a multiple of this value has a reflectanceof substantially 0%, which means that no light is reflected from thethin film layer 4. Therefore, if the thickness T deviates from thesevalues, the reflected light increases.

The ideal case is such that the thickness T of the thin film layer 4 andthe natural number k satisfy:T=(2k−1)×λ/(4×n ₃)This case provides conditions under which the reflected light 8 and thereflected light 9 interfere with and intensify each other most, in whicha quantity of light reflected from the thin film layer 4 increases tonot less than 10 times that of the Fresnel reflection. This makes itpossible to observe the interference fringes formed by the reflectedlight from the thin film layer 4 and the reflected light from thesurface of the waveguide substrate 1, and to evaluate the thicknessuniformity of the waveguide substrate 1 during polishing by utilizingthe interference fringes. The polishing is performed appropriatelyaccording to the interference fringes thus observed by, for instance,varying the pressure distribution upon polishing so that thenon-uniformity is reduced. Thus, the uniform thinning of the waveguidesubstrate 1 is enabled.

Using the foregoing method, the inventors of the present inventionperformed the polishing by keeping a state in which not more than oneinterference fringe was observed with in a plane, while measuring anabsolute value of a thickness of the waveguide substrate 1 by the leveldifference meter, and finally they successfully thinned the waveguidesubstrate 1 to a thickness of 3.5 μm with a thickness variation of notmore than 300 nm. It should be noted that in the interference fringeobservation using a light source with a wavelength of 632.8 nm, it canbe determined that the non-uniformity is approximately 300 nm per oneinterference fringe. It was confirmed by actual measurement that aridge-type optical waveguide as shown in FIG. 1, formed by dry-etchingthe waveguide substrate 1 thus thinned, had excellent optical waveguidecharacteristics. Further, it is possible to produce a wavelengthconverter by forming a cyclic polarity inversion structure and anoptical waveguide in a thinned substrate. Since this wavelengthconverter has an enhanced thickness uniformity of the optical waveguide,an excellent phase matching state is achieved, whereby a high wavelengthconversion efficiency is achieved.

It should be noted that the interference occurs between the reflectedlight 8 and the reflected light 9 that causes the lights to cancel eachother and consequently minimizes the reflected light from the thin filmlayer 4 when the thickness T satisfies:T=(k×λ)/(2×n ₃)where k represent a natural number. This is because there is a phasedifference of π/2 between a phase of the reflected light at an interfaceof one side of the thin film layer 4 and a phase of the reflected lightat an interface of the other side of the thin film layer 4. It should benoted that this applies in the case where n₁>n₃ and n₂>n₃ are satisfied.

When n₁>n₃>n₂ is satisfied, the reflected light is minimized when Tsatisfies T=((2k−1)×λ)/(4×n₃).

In other words, when n₁>n₃ and n₂>n₃ are satisfied, T should be set soas to satisfy:T≠(k×λ)/(2×n ₃).

When n₁>n₃>n₂ is satisfied, T should be set so as to satisfy:T≠((2k−1)×λ)/(4×n ₃).

When the thickness T of the thin film layer 4 is in a range of ±30 nmfrom the value satisfying T=((2k−1)×λ)/(4×n₃) in particular, it ispossible to limit the reduction of the intensity of the reflected lightfrom the thin film layer 4 to not more than 5% from the maximum value.Therefore, by setting the thickness T of the thin film layer 4 in theforegoing range, the observation of the interference fringes isfacilitated. Thus, it is preferable to perform the control of thicknessin this range. It should be noted that if the reflected lights 8 and 9have extremely low intensities, reflected light from the thin film layer4 still has a low intensity even if the reflected lights 8 and 9interfere with and intensify each other. However, it has been found byexperiments that interference fringes can be observed in the case whereat least one of a difference between the refractive indices n₁ and n₃and a difference between the refractive indices n₂ and n₃ is not lessthan 0.05.

Embodiment 2 is described thus by referring to a case where the thinfilm layer 4 is a dielectric single-layer film containing Ta₂O₅ as aprincipal component, but the material of the thin film layer 4 is notlimited to this. Besides, the same principle applies also in the casewhere the thin film layer 4 is a multilayer film. For instance, in thecase where a multilayer film composed of a plurality of layers havingdifferent refractive indices is used as the thin film layer 4,low-reflection conditions and high-reflection conditions can be derivedfrom refractive indices and thicknesses of the respective layers, asgenerally known.

For instance, as described above, on the waveguide substrate 1 made ofMgO:LN crystal (having a refractive index of 2.17 with respect to lighthaving a wavelength of 632.8 nm), two kinds of thin films containingTa₂O₅ as a principal component, which have refractive indices of 2.05and 2.10 respectively with respect to light having a wavelength of 632.8nm, may be formed so as to compose a multi-layer film. The layers areconfigured so that layers having a refractive index of 2.05 are incontact with the waveguide substrate 1 and the base substrate 2. A thinfilm of one of the kinds that has a refractive index of 2.05 and a thinfilm of the other kind that has a refractive index of 2.10 are formedalternately so that the former has a thickness of 77 nm and the latterhas a thickness of 75 nm. A multi-layer film composed of nine layersthus formed as the thin film layer 4 has a reflectance of 2.3%, while amulti-layer composed of nineteen layers as the thin film layer 4 has areflectance of 7.1%. Thus, by controlling refractive indices andthicknesses of the respective layers of the multi-layer film, thereflectance can be controlled.

It should be noted that the low-reflection condition is defined as acondition under which reflected lights from interfaces of the respectivelayers have different phases and cancel one another, thereby decreasingan intensity of a reflected light. On the other hand, thehigh-reflection condition is defined as a condition under whichreflected lights at interfaces of the respective layers are synthesizedwith one another, thereby increasing an intensity of a reflected light.By designing the thin film layer 4 according to the high-reflectioncondition, it is possible to maximize the reflected light from the thinfilm layer 4 as a whole.

It should be noted that Embodiment 2 is described with reference to acase where the light source for the interference fringe observation hasa wavelength λ of 632.8 nm. However, examples of a light sourcegenerally used for the measurement purpose have wavelengths of 380 nm,410 nm, 441.6 nm, 488 nm, 532 nm, etc. With respect to any one of thesewavelengths, the optimal thickness of the thin film layer 4 can bedetermined by the same calculation, and the reflected light quantityfrom the thin film layer 4 can be increased.

EMBODIMENT 3

The following will describe an optical element according to Embodiment 3of the present invention, while referring to the drawings. In thefollowing description, an optical waveguide-type element produced bydirect bonding is taken as an example of an optical element, in which asubstrate formed with LiNbO₃ crystal (hereinafter abbreviated as LNcrystal), which is a ferroelectric crystal, and a substrate formed withMgO-doped LiNbO₃ crystal (hereinafter abbreviated as MgO:LN crystal) areused as two substrates to be bonded by direct bonding. However, thepresent invention is not limited to this configuration.

An optical element according to Embodiment 3 is configured so that anon-bonding region is present in an optical element composed of twosubstrates (bonded paired substrates) having been subjected to opticalpolishing, the non-bonding region being a gap present at a part of abonding surface.

The optical element according to Embodiment 3 has a characteristic inthat a gap (non-bonding region) is provided at a part of a bondingsurface when substrates of a same kind or different kinds are bonded soas to form the optical element. With this configuration, a refractiveindex difference is obtained in a substrate thickness direction.Therefore, for instance, by thinning one of the substrates bonded andthereafter forming a ridge structure so as to form an optical waveguide,an optical waveguide-type element can be produced irrespective ofrefractive indices of the substrates.

An optical element according to Embodiment 3 of the present invention isdescribed below, with reference to the drawings. FIG. 8 is a perspectiveview illustrating a structure of the optical element according toEmbodiment 3 of the present invention. The optical element of Embodiment3 is configured so that a refractive index difference is obtained in thevicinity of a bonding surface between a base substrate 22 and awaveguide substrate 21 by providing a non-bonding region 24, in place ofthe thin film layer 4 in Embodiments 1 and 2. This configuration allowsfor the waveguiding of light, irrespective of a material used forforming the waveguide substrate 21, and enhances the waveguidecharacteristics. Therefore, it improves the selectivity of the materialof the substrate.

For instance, a X-cut MgO:LN crystal substrate is used as the waveguidesubstrate 21. Further, a X-cut LN crystal substrate is used as the basesubstrate 22. Still further, the waveguide substrate 21 has a ridgestructure so that a ridge-type optical waveguide 23 is formed. Theoptical waveguide 23 is not in contact with the base substrate 22, but agap is provided therebetween to function as the non-bonding region 24.Principal surfaces of the waveguide substrate 21 and the base substrate22 are subjected to optical polishing. Further, a surface of the basesubstrate 22 on the waveguide substrate 21 has a non-bonding region 24that is recessed. The non-bonding region 24 is to form a gap when thebase substrate 22 and the waveguide substrate 21 are bonded with eachother. After the waveguide substrate 21 and the base substrate 22 arebonded, the gap formed by the non-bonding region 24 separates them fromeach other.

The following will describe a method for producing an optical elementaccording to Embodiment 3. First, the non-bonding region 24 is formed onthe base substrate 22. Various methods are available for forming thenon-bonding region 24, and one example is dry-etching. On a principalsurface of the base substrate 22 on one side, a Cr film is formed tohave a thickness of 200 nm by RF sputtering or electron beam vapordeposition (EB vapor deposition), and a region where the non-bondingregion 24 is to be formed is patterned by photolithography and wetetching. Thereafter, the base substrate 22 is subjected to dry etchingusing the Cr film as an etching mask, whereby an etching groove with adepth of 100 nm to 300 nm is formed as the non-bonding region 24.Thereafter, Cr used as a mask is removed by wet etching. It should benoted that the base substrate 22 may be formed by a method other thanthe foregoing method.

A principal surface of the waveguide substrate 21 and the principalsurface of the base substrate 22 on which the non-bonding region 24 isformed, which are surfaces to be bonded directly, are subjected to atreatment for imparting hydrophilicity. More specifically, principalsurfaces of the waveguide substrate 21 and the base substrate 22 to besubjected to direct bonding are subjected to acetone ultrasoniccleaning, and the substrates are immersed in a mixture solution ofammonia water (aqueous ammonia), hydrogen peroxide, and deionized waterat a ratio of 1:1:6 (hereinafter referred to as ammonia-hydrogenperoxide solution) for not less than fifteen minutes, rinsed withdeionized water, and thereafter, dried. It should be noted that normalammonia water having a concentration of 20% to 40% may be used.Desirably, the ammonia water has a concentration of 30%.

Subsequently, the waveguide substrate 21 and the base substrate 22 arecombined with each other so that directions of respectivecrystallographic axes of the waveguide substrate 21 and the basesubstrate 22 coincide with each other and that the surfaces thereofimparted with hydrophilicity are brought into contact. By applying aslight pressure thereto, the waveguide substrate 21 and the basesubstrate 22 are brought into close contact with each other, except fora portion where the non-bonding region 24 is formed.

Further, the waveguide substrate 21 and the base substrate 22 thusbrought into close contact are subjected to a heat treatment, whereby adirect-bond substrate is obtained. The heat treatment is carried out at350° C. to 800° C., with use of an oven, at a temperature rising rateof, for instance, 50° C./h to 500° C./h for preventing the delaminationof the bonded substrates and damages due to rapid temperature rise.

To form the optical waveguide 23 in the direct-bond substrate thusobtained, the waveguide substrate 21 is subjected to surface polishingso as to be thinned, and finally has a thickness of 3 μm to 4 μm.Thereafter, a patterning corresponding to the optical waveguide 23 iscarried out by photolithography, and a ridge of 1.5 μm to 2 μm is formedby dry-etching on the surface of the waveguide substrate 21 thusthinned, which is a MgO:LN crystal substrate. Thus, the ridge-typeoptical waveguide 23 is formed. It should be noted that both end facesof the optical waveguide 23 are mirror-finished after the ridge isformed.

It should be noted that the direct-bond substrate without a ridgewaveguide being formed therein is applicable as an optical element suchas a diffraction grating, a modulator, a deflector, etc.

Here, the non-bonding region 24 is described in more detail. Generally,the sufficient trapping of light in an optical waveguide and thewaveguiding with low losses are regarded as the most importantcharacteristics that a light-waveguide-type optical element is requiredto possess. Further, in order to make the transmission characteristic ofan optical waveguide uniform through an entirety of the opticalwaveguide and to enhance a production yield, it is necessary to securethe uniformity in controlling a shape of the optical waveguide. In thecontrol of the optical waveguide shape, in the case where one of thesubstrates directly bonded is thinned and an optical waveguide is formedtherein, it is essential particularly that the thinned substrate has ahigh thickness uniformity.

A bonding surface 25 is one of principal surfaces of the waveguidesubstrate 21 at which the waveguide substrate 21 is in contact with thebase substrate 22. In the bonding surface 25, the waveguide substrate 21and the base substrate 22 partly are not in contact with each other.This is because the optical element according to Embodiment 3 has a gapas the non-bonding region 24 between the waveguide substrate 21 and thebase substrate 22. Therefore, in the bonding surface 25, the opticalwaveguide 23 may be formed within a range where the gap is formed,whereby a refractive index difference can be caused between the opticalwaveguide 23 and the gap. Thus, this easily causes the optical waveguide23 to have a sufficient light trapping effect in the substrate thicknessdirection. Here, if a portion bonded with the base substrate 22 ispresent in a region of the bonding surface 25 defined by projecting theoptical waveguide 23 perpendicularly to the bonding surface 25, theoptical waveguide 23 has an insufficient light trapping effect, andtransmission losses of guided light increase. In other words, it isimportant that the non-bonding region 24 has to encompass the regiondefined by projecting the optical waveguide 23 to the bonding surface25, or has to be broader than that. By so doing, a sufficient lighttrapping effect due to the refractive index difference can be achieved.

The process for forming the optical waveguide 23 is performed afterthinning the waveguide substrate 21. By providing the non-bonding region24, the alignment of the optical waveguide 23 with the non-bondingregion 24 can be carried out easily by referring to the non-bondingregion 24 as a reference, in the patterning process for forming theoptical waveguide 23 by photolithography. Further, by forming thenon-bonding region 24 by referring to the crystallographic axis of thebase substrate 22, for instance, the optical waveguide 23 can be formedin conformity with the crystallographic axis of the base substrate 22.This enables the suppression of waveguide losses.

Further, the following describes the width of the non-bonding region 24in the same direction as the width direction of the optical waveguide23, and the interval of the non-bonding regions 24 in the widthdirection of the optical waveguide 23. In the case where a plurality ofnon-bonding regions 24 are formed, they should be formed at sufficientintervals so that a sufficient bond strength is secured for the processafter the direct bonding. It has been confirmed by actual measurementthat in the case where the non-bonding regions 24 are formed with aninterval of not less than 1 mm between centers of adjacent non-bondingregions 24, a width of 1 μm to 500 μm of the non-bonding region 24ensures a sufficient direct bond strength. Further, it also has beenconfirmed by actual measurement that in the case where the non-bondingregions 24 are formed with an interval of 30 μm to 1 mm between centersof adjacent non-bonding regions 24, a width of not more than 30 μm ofthe non-bonding region 24 ensures a sufficient direct bond strength.Further, when the optical waveguide 23 has a width of 5 μm, thenon-bonding region 24 desirably has a width of 10 μm to 30 μm. Stillfurther, when the non-bonding region 24 has a width of 30 μm, aninterval between centers of the non-bonding regions 24 desirably is notless than 100 μM.

In the case where a plurality of optical waveguides 23 with a width of 3μm each are formed, it is preferable that the width and the interval ofthe non-bonding regions 24 are set to be 5 μm to 10 μm and several 10μm, respectively, considering the yield and the mass production of theoptical element, and the characteristics of the optical waveguide 23.

Further, in the case where the non-bonding regions 24 are formed in adirection on the bonding surface, the load on the substrate uponmachining (for instance, cutting or polishing) of the bonded substratetends to be biased in the direction in which the non-bonding regions 24are formed. This sometimes leads to damage to the optical element uponcutting or polishing, for instance. To prevent this problem, thenon-bonding regions 24 preferably are provided in a lattice form asshown in FIG. 9.

FIG. 9 is an orthographic drawing by the third angle projection methodillustrating positions at which the non-bonding regions 24 are arranged.As shown in the plan view of FIG. 9, non-bonding regions 24 are arrayednot only in the direction along the optical waveguide 23 but also in adirection perpendicular to the optical waveguide 23. Further, all theintervals of the lattice are equal. In other words, the non-bondingregions 24 are formed in a lattice form of equal intervals. Thisconfiguration distributes and reduces the load upon cutting orpolishing, thereby allowing for improved bond strength. In thisconfiguration, machining resistance was measured with the density of thebonding regions varied, and it was found that a high machiningresistance was obtained. By patterning the non-bonding regions 24 in alattice form of equal intervals in a direct-bonding surface, themachining resistance of the direct-bond substrate can be increased.

Next, the following describes a depth (gap depth) of the non-bondingregion 24 in the substrate thickness direction. As described above,since the optical waveguide 23 has a ridge structure, the light trappingeffect is sufficient in the width direction of the optical waveguide 23and on the opposite side of the base substrate 22. Further, it isnecessary to make sure that the guided light does not leak, either,through the non-bonding region 24 side of the optical waveguide 23. Inother words, the non-bonding region 24 has to have a gap depth such thatno leaked portion of light propagating through the optical waveguide 23is present in the base substrate 22.

Therefore, optical elements were produced according to Embodiment 3 witha gap width of 0.005 μm to 0.5 μm, and characteristics of the elementswere determined. As a result, in the case where the gap depth was notless than 0.01 μm, no deterioration was observed in the light trappingeffect in the thickness direction of the optical waveguide 23.Therefore, in a state in which the non-bonding region 24 is filled withair, a gap depth of the non-bonding region 24 of not less than 0.01 μmsuffices to allow the guided light to propagate through the opticalwaveguide 23 sufficiently. However, as described above, by forming thenon-bonding region 24 by dry-etching, it is possible to control the gapdepth with a high precision of several %.

It should be noted that the use of substrates with high parallelism(thickness uniformity) as the waveguide substrate 21 and the basesubstrate 22 enables the thickness control with high precision in thepolishing and thinning of the waveguide substrate 21 also. For instance,the thickness uniformity of the waveguide substrate 21 thinned to athickness of 3 μm was controlled so as to be within a range of ±50 nmwithin a 3-inch wafer plane.

Further, when the waveguide substrate 21 and the base substrate 22 arebonded by direct bonding, the smoothness degrees of the surfaces to bebonded are significant. For instance, when the surface to be bonded hasa surface roughness of 5 nm or more, the bonding is difficult.Therefore, in the case where the film forming, the etching, or anotherprocess is carried out before the waveguide substrate 21 and the basesubstrate 22 are bonded directly, smoothnesses of the surfaces to bebonded may be deteriorated, which is not preferable. However, it wasconfirmed by actual measurement that virtually no deterioration occurredin the smoothness of principal surfaces of the base substrate 22 havingbeen subjected to a process such as Cr sputtering, photolithography,wet-etching, dry-etching, etc. as described above, and therefore, thedirect bonding of the foregoing base substrate 22 with the waveguidesubstrate 21 was achieved readily.

Further, a concern also arises about the possible damage to the air gapportion (the non-bonding region 24) in the polishing and thinningprocess with respect to the waveguide substrate 21, as well as about thepossible deterioration of bond strength, that is, the delamination uponpolishing due to the presence of the non-bonding regions 24. However, itwas also confirmed that the waveguide substrate 21 and the basesubstrate 22 that were bonded directly did not exhibit any delaminationor deterioration even in the process after the direct bonding (polishingand thinning of the waveguide substrate 21, photolithography,dry-etching, end-face polishing, etc.), and the excellent bonding statewas maintained.

Furthermore, it was also confirmed that the optical waveguide 23 in theoptical element according to Embodiment 3 had excellent waveguidecharacteristics, and a sufficient light trapping effect and waveguidingwith low transmission losses were achieved.

It should be noted that in a configuration example of an optical elementin which the waveguide substrate 21 and the base substrate 22 are bondeddirectly and which has the non-bonding region 24, the non-bonding region24 is formed by forming a recess on the base substrate 22, but thenon-bonding region 24 may be formed by forming a recess on the waveguidesubstrate 21. This configuration can be formed readily, and achieves thesame effects.

Further, materials used for forming the waveguide substrate 21 and thebase substrate 22 are not limited, and the provision of the non-bondingregion allows an optical element to be formed by bonding substrates ofdifferent kinds or a same kind by direct bonding without limitingrefractive indices of the substrates. It should be noted that a bondingmethod other than the direct bonding may be used for bonding substrates.

FIG. 10 is a perspective view illustrating another configuration of anoptical element according to Embodiment 3. As shown in FIG. 10, theoptical element may be configured so that an optical waveguide 33 isarranged within a non-bonding region 34. For this configuration, awaveguide substrate 31 on which a ridge-type optical waveguide 33 isformed beforehand, and a base substrate 32 having a non-bonding region34 that is obtained by forming a recess, are prepared. For instance, anX-cut MgO:LN crystal substrate and an X-cut LN crystal substrate areused as the waveguide substrate 31 and the base substrate, respectively.Principal surfaces of the waveguide substrate 31 and the base substrate32 are subjected to optical polishing. The optical waveguide 33 ispresent within the non-bonding region 34 that is provided by forming arecess in the base substrate 32, and a gap also is present therein,separating the optical waveguide 33 from the base substrate 32. Withthis configuration, an excellent direct bonding state and excellentwaveguide characteristics of the optical waveguide 33 were obtained.

It should be noted that though the optical element according toEmbodiment 3 is described as an optical element of an optical waveguidetype, but it is not limited to an optical element of an opticalwaveguide type. It is possible to, for instance, form cyclic non-bondingregions in a part of the bonding surface, so as to make the element adiffractive optical element.

EMBODIMENT 4

The following will describe an optical element according to Embodiment 4while referring to the drawings. FIG. 11 is a perspective viewillustrating the optical element according to Embodiment 4. The opticalelement according to Embodiment 4 is obtained by filling the non-bondingregion of the optical element of Embodiment 3 with a material differentfrom materials used for forming the substrates to be bonded, forinstance, ultraviolet (UV) curable resin. By so doing, the bonding isreinforced by the adhesiveness of the UV curable resin, in addition tothe bond strength between the substrates bonded directly. This furtherenhances the machining resistance during the process subsequent to thebonding (thinning of the direct-bond substrate by polishing, etc.).

In FIG. 11, a X-cut MgO:LN crystal substrate as a waveguide substrate 41and a X-cut LN crystal substrate as a base substrate 42 are bonded witheach other. A non-bonding region 44 is formed by forming a recess on thebase substrate 42, which is filled with an UV curable resin 47. Further,principal surfaces of the waveguide substrate 41 and the base substrate42 are subjected to optical polishing. In FIG. 11, a plurality ofnon-bonding regions 44 are formed, but the number of the same is notlimited to this. It should be noted that in the optical element, thewaveguide substrate 41 may be configured to have a refractive indexhigher than that of the UV curable resin 47 filled in the non-bondingregion 44 and that of the base substrate 42 so that the waveguidesubstrate 41 functions as a waveguide, or refractive indices may bevaried so that selective waveguiding is achieved. Furthermore, thewaveguide substrate 41 may be processed so as to have a ridge structure,to function as a ridge waveguide.

The method for producing the optical element according to Embodiment 4is identical to that of the optical element according to Embodiment 3partway. More specifically, for instance, a Cr film is formed on thebase substrate 42 by RF sputtering or EB vapor deposition, patterned byphotolithography or wet-etching, and subjected to dry-etching, so thatthe non-bonding regions 44 are formed. Thereafter, the waveguidesubstrate 41 and the base substrate 42 are subjected to a treatment forimparting hydrophilicity, brought into close contact with each other,and subjected to a heat treatment, whereby they are bonded directly.This is followed by a process different from that in Embodiment 3.

An UV curable resin 47 is filled in a gap formed as the non-bondingregion 44 in the direct-bond substrate obtained by the foregoingproducing process. Here, the UV curable resin 47 flows into thenon-bonding region 44 formed in the direct-bond substrate by capillaryaction. In the case where an UV curable resin with a low viscosity(approximately 60 cp or less) is used as the UV curable resin 47 inparticular, the speed of the capillary action increases significantly,whereby the filling is competed within a short period of time moreeasily. Thereafter, ultraviolet rays are projected to the direct-bondsubstrate from outside its surface. This cures the UV curable resin 47filled, and causes a strong bonding force to be achieved between thewaveguide substrate 41 and the base substrate 42. Thereafter, a ridgestructure may be formed to make the optical element an optical waveguidetype. Alternatively, the optical element may be processed, or may beused as it is, according to its purpose of use.

Further, the optical element according to Embodiment 4 has an effect ofdistributing and reducing a load on the substrates since the UV curableresin 47 functions as a buffer upon machining. Therefore, there is lesspossibility of damage to the substrates or delamination at the bondedportions subjected to direct bonding even upon the polishing andthinning of the waveguide substrate 41, for instance.

Still further, in the case where the optical element according toEmbodiment 4 is made to be, for instance, an optical element of anoptical waveguide type, the material filled in the non-bonding region 44prevents any foreign matter to the gap from entering the gap andcontacting the waveguide substrate 41. Therefore, the waveguidecharacteristics do not deteriorate.

Another configuration example of the optical element according toEmbodiment 4 is shown in FIG. 12. As shown in FIG. 12, the waveguidesubstrate 41 and the base substrate 42 may be bonded with each otherwith a thin film layer 45 interposed therebetween. Since the highlyprecise and uniform film thickness control can be achieved with respectto a thin film, if a dielectric is used for forming a thin film, it ispossible to achieve various refractive indices and absorptioncoefficients by selecting the material.

A method of producing the optical element shown in FIG. 12 includes thebonding of the base substrate 42 having the non-bonding region 44 formedthereon with the waveguide substrate 41, like in the method forproducing the optical element shown in FIG. 11. Here, the thin filmlayer 45 is formed beforehand on the waveguide substrate 41 in the samemanner as that for forming the thin film layer 4 of the optical elementin Embodiments 1 and 2. It should be noted that the thin film layer 45preferably has a refractive index smaller than those of the waveguidesubstrate 41 and the base substrate 42, for instance, a single-layerfilm containing Ta₂O₅ as a principal component.

The base substrate 42 having the non-bonding region 44 and the waveguidesubstrate 41 having the thin film layer 45 are combined so that the thinfilm layer 45 and the side of the base substrate 42 on which thenon-bonding region 44 is formed are bonded directly. Thereafter, an UVcurable resin 47 is filled in the non-bonding region 44. Subsequently,UV rays are projected to the direct-bond substrate from outside thesurface, whereby the UV curable resin 47 filled is cured.

Still another configuration example of the optical element according toEmbodiment 4 is shown in FIG. 13. As shown in FIG. 13, the opticalelement may be configured so that the thin film layer 45 and thenon-bonding region 44 are selectively formed in the same layer on thebase substrate 42, and the waveguide substrate 41 is formed thereon. Theforegoing configuration is obtained by forming the thin film layer 45 onthe waveguide substrate 41 or the base substrate 42 by sputtering or thelike, patterning the same by photolithography or dry-etching, andremoving the thin film layer 45 partially. A portion where the thin filmlayer 45 is removed becomes as the non-bonding region 44. After bondingthe waveguide substrate 41 and the base substrate 42 by direct bonding,the UV curable resin 47 is filled in the non-bonding region 44.

As described above, even in the case where the waveguide substrate 41and the base substrate 42 are bonded with the thin film layer 45interposed therebetween, it is possible to enhance the bond strength byremoving the thin film layer 45 partly to provide the non-bonding region44, filling the UV curable resin 47 therein, and curing the same.

It should be noted that though the material to be filled in thenon-bonding region 44 is an UV curable resin herein, the material is notlimited to this.

EMBODIMENT 5

The following will describe an optical element according to Embodiment 5of the present invention while referring to the drawings. FIGS. 14A to14D are front views illustrating steps for producing an optical elementaccording to Embodiment 5 successively. The optical element ofEmbodiment 5 is identical to the optical element of Embodiment 3 exceptthat a dielectric layer is formed in the non-bonding region.

FIG. 14D is a front view illustrating a state of the optical elementfinished. In FIG. 14D, an X-cut MgO:LN crystal substrate as a waveguidesubstrate 51 and an X-cut LN crystal substrate as a base substrate 52are bonded with each other. In a recessed region on the base substrate52, a dielectric layer 55 is formed. The dielectric layer 55 is, forinstance, a single-layer film containing Ta₂O₅ as a principal component.

The following will describe a method for producing an optical elementaccording to Embodiment 5. First of all, as shown in FIG. 14A, anon-bonding region 54 is formed on the base substrate 52. Then, the thinfilm layer 55 is deposited on a surface of the base substrate 52 bysputtering as shown in FIG. 14B. Since the thin film layer 55 is formedon the base substrate 52, only a small bond strength is achieved if thedirect bonding is carried out in this state, and the machiningresistance of the thin film is insufficient. Therefore, as shown in FIG.14C, a state is created in which the thin film layer 55 is depositedonly in the non-bonding region 54. More specifically, the thin filmlayer 55 is removed by polishing except for the portion in thenon-bonding region 54 using a chemical mechanical polishing (CMP)device, so as to smoothen a surface of the base-substrate 52 includingthe thin film layer 55. It should be noted that the CMP device is apolishing device with extremely high precision, known as a devicecapable of achieving the absolute polishing degree control at asubmicronic or more minute level and the surface smoothness at theprecision of not more than several 10 nm.

The depth of the non-bonding region 54 is set to be 100 nm to 300 nm,and the thickness of the thin film layer 55 deposited is set to be 150nm to 350 nm. The CMP process is controlled so that the principalsurface of the base substrate 52 is trimmed by approximately 50 nm. Thiscauses the surface of the base substrate 52 to be exposed completely,and the surface of the thin film layer 55 and the surface of the basesubstrate 52 to be smoothened. It should be noted that by depositing thethin film layer 55 so that its thickness exceeds the depth of thenon-bonding region 54, it is possible to smooth the surface of the thinfilm layer 55 and the surface of the base substrate 52 by the CMPprocess. Since the use of the CMP device makes it possible to carry outthe foregoing polishing and to apply the mirror-finishing also to thepolished surface simultaneously, an effect of omitting a separate stepof applying the mirror-finishing for the direct bonding can be achieved.

As shown in FIG. 14D, the base substrate 52 and the waveguide substrate51 are combined so that crystallographic axis directions of thesubstrates coincide with each other and that the principal surface ofthe base substrate 52 having the non-bonding region 55 thereon and theprincipal surface of the waveguide substrate 51 are brought into contactwith each other, and a slight pressure is applied thereto. Thus a closecontact state is created, and the substrates are subjected to a heattreatment so as to be bonded directly. The base substrate 52 polished bythe CMP device, when bonded with the waveguide substrate 51 by directbonding, ensures creation of a close contact state and a direct-bondstate at substantially the same level as that of the direct bonding ofnormal substrates.

In a bond substrate thus obtained also, since a high substrate bondingprecision by direct bonding is achieved, it is possible to achievesimultaneously both of the significant improvement of machiningresistance because of the direct-bond state of the base substrate 52 andthe waveguide substrate 51, and the multifunctionality and highperformance (low losses, multifunctionality) of the optical elementbecause of the possession of the dielectric thin film, as compared withthe configuration of Embodiment 1 in which only the thin film layer andthe base substrate are bonded directly.

It should be noted that the thin film layer 55 may be formed partly inthe non-bonding region 54. For instance, as shown in FIGS. 15A and 15Bin an optical element in which an optical waveguide 53 is formed, thethin film layer 55 is formed in the waveguide substrate 51, so as to beat least formed in a surface region defined by perpendicularlyprojecting the optical waveguide 53 thereto. With this configuration,guided light is trapped sufficiently in the waveguide. Thisconfiguration reduces a volume of the thin film layer 55, whereby theproduction cost can be reduced.

The following will describe a method for forming the optical waveguideshown in FIGS. 15A and 15B. First, the thin film layer 55 is formed on asurface of the waveguide substrate 51 opposite to the surface on whichthe optical waveguide 53 is formed. Then, the thin film layer 55 ispatterned by photolithography, dry-etching, etc., so that the thin filmlayer 55 is formed at least exactly below the optical waveguide 53.Finally, the waveguide substrate 51 and the base substrate 52 are bondeddirectly. With this process, it is possible to produce an opticalelement without polishing. It should be noted that a resin or the likemay be filled in a space 54 a in the non-bonding region 54. Thisreinforces the bonding surface.

EMBODIMENT 6

The following will describe an optical element according to Embodiment 6of the present invention while referring to the drawings. The opticalelement according to Embodiment 6 is configured so that a plurality ofsubstrates having been subjected to optical polishing are stacked, andthe substrates are bonded with one another by direct bonding. Further,non-bonding regions are arranged cyclically in each substrate.

FIG. 16 is a perspective view of the optical element according toEmbodiment 6 of the present invention. The optical element of Embodiment6 is formed by repetitively carrying out the following process, so as toform a stacked structure. The process includes bonding a substratehaving non-bonding regions with another substrate by direct bonding,thinning the substrate thus bonded, and forming non-bonding regions. Abase substrate 62 a includes non-bonding regions 64 a that are groovesformed in parallel with one another at regular intervals. On the basesubstrate 62 a, bar-form substrates 62 b are formed in parallel with oneanother at regular intervals, so as to cross the non-bonding regions 64a perpendicularly. A groove-like non-bonding region 64 b is formedbetween each pair of adjacent bar-like substrates 62 b. Further, on thesubstrates 62 b, a plurality of bar-like substrates 62 c are formed inparallel with one another at regular intervals, so as to cross thenon-bonding regions 64 b perpendicularly. A groove-like non-bondingregion 64 c is formed between each pair of adjacent bar like substrates62 b.

The following will describe a method for producing the optical elementof Embodiment 6. Specifically, first of all, non-bonding regions 64 aare formed on a LN crystal substrate as the base substrate 62 a bydry-etching using a Cr film formed by sputtering, for instance, as anetching mask. After the patterning of Cr, the base substrate 62 a issubjected to dry-etching using Cr as an etching mask, so thatstripe-like etching grooves (non-bonding regions) with a depth of 3 μmare formed at regular intervals. It should be noted that the patterningby photolithography enables the patterning of a desired pattern withhigh precision. For instance, the non-bonding regions can be formed in apattern of cyclically arranged polygons. More specifically, an opticalelement as shown in FIG. 17 can be formed. In other words, it ispossible to form an optical element in which substrates 62 d and 62 ehaving cyclically arranged hexagonal etching grooves (non-bondingregions 64 d and 64 e) are bonded so that a cycle of hexagons of onesubstrate is shifted by a half cycle from a cycle of hexagons of theother substrate.

On the other hand, non-bonding regions 64 b that are 3 μm-wide etchinggrooves are formed on one principal surface of a LN crystal substrate asthe substrate 62 b in the same manner as that described above. It shouldbe noted that a direction of etching grooves of the substrate 62 b isdetermined considering that the base substrate 62 a and the substrate 62b are bonded so that directions of etching grooves of the base substrate62 a and the substrate 62 b cross each other perpendicularly.Thereafter, Cr used as a mask is removed by wet-etching, and surfaces tobe bonded, that is, the principal surface of the base substrate 62 a onwhich the non-bonding regions 64 a are formed, and the principal surfaceof the substrate 62 b on which the non-bonding regions 64 b are formed,are subjected to a treatment for imparting hydrophilicity. Thereafter,the base substrate 62 a and the substrate 62 b are combined so thatcrystallographic axis directions of the substrates coincide with eachother and so that the principal surface of the base substrate 62 ahaving the non-bonding regions 64 a and the principal surface of thesubstrate 62 b having the non-bonding regions 64 b are brought intocontact with each other, whereby a state of close contact between thebase substrate 62 a and the substrate 62 b is formed. Further, thesubstrates in close contact are subjected to a heat treatment, so thatthe base substrate 62 a and the substrate 62 b are bonded directly,whereby a direct-bond substrate is obtained.

In the direct-bond substrate thus obtained, the substrate 62 b ispolished and thinned so as to have a thickness of 2.5 μm. By so doing,the substrate 62 b is modified from a single substrate into a pluralityof bar-like substrates. Further, by the same technique as that describedabove, a substrate 62 c on which etching grooves as non-bonding regions64 c are formed is bonded directly with the plurality of bar-likesubstrates 62 b so that the non-bonding regions 64 b and the non-bondingregions 64 c cross each other perpendicularly, and is polished andthinned. This process is carried out repetitively, whereby substratesare stacked. In other words, non-bonding regions are formed on asubstrate, the substrate is bonded directly with another substrate sothat a non-bonding-region-formed surface is in contact with the anothersubstrate, and the substrate thus bonded directly is polished andthinned, so that a plurality of bar-like substrates are formed.

Through the foregoing process, a crystal substrate having a periodicallystacked structure is formed. This is called a photonic crystal, which isa medium having a structure in which the refractive index cyclicallyvaries, and which is capable of controlling lightwave. A photoniccrystal has a characteristic of possessing a band structure with respectto lightwave, and draws attention as being capable of providing aspecific waveguiding control. Generally, the production of photoniccrystal is carried out by electron beam exposure, so that electron holeswith a diameter of several 100 nm to several 100 μm each are arrangedcyclically in a crystal. Therefore, the production of a photonic crystalrequires fine micromachining, and has been regarded very difficult.

According to the optical element producing method according toEmbodiment 6, the formation of non-bonding regions by dry-etching makesit possible to form cyclically arranged non-bonding regions. Further, itis possible to produce an optical element in which the size of thenon-bonding regions, the interval thereof, and the thickness thereof inthe substrate thickness are in the submicronic order to several 10 μm.Therefore, it is possible to produce a photonic crystal by the opticalelement producing method according to Embodiment 6. Furthermore,generally, a photonic crystal is formed with a polycrystalline materialthat requires the strict control of the composition and the crystallinestructure, but according to Embodiment 6, the structure can be producedusing a homogeneous monocrystalline material.

It should be noted that materials composing optical elements andstructures of the same in the embodiments described above are merelyexamples, and the present invention is not limited to these specificexamples. The method for bonding substrates is not limited to the directbonding, either.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. An optical element, comprising: a base substrate; a waveguidesubstrate; and a thin film layer provided between the base substrate andthe waveguide substrate, having a single-layer structure or a multilayerstructure including a film containing Ta₂O₅ or Nb₂O₅ as a principalcomponent, wherein the waveguide substrate is made of a crystal that istransparent with respect to light having a wavelength λ and that has arefractive index n₁ with respect to the light having the wavelength λ,the base substrate is made of a material having a refractive index n₂with respect to the light having the wavelength λ, the thin film layeris made of a material having a refractive index n₃ with respect to thelight having the wavelength λ, and has a thickness T satisfying, whenn₁>n₃ and n₂>n₃, T≠(k×λ)/(2×n₃), and when n₁>n₃>n₂, T≠((2k−1)×λ)/(4×n₃)where n₃≠n₁≠n₂, and k represents a natural number.
 2. The opticalelement according to claim 1, wherein at least one of bonding betweenthe base substrate and the thin film layer and bonding between thewaveguide substrate and the thin film layer is direct bonding.
 3. Theoptical element according to claim 1, wherein the waveguide substrate ismade of LiNb_(x)Ta_((1-x))O₃(0≦x≦1).
 4. The optical element according toclaim 1, wherein an optical waveguide is formed in the waveguidesubstrate.
 5. The optical element according to claim 1, wherein the thinfilm layer has a thickness of not less than 50 nm.
 6. The opticalelement according to claim 2, wherein the thin film layer includes afilm containing Ta₂O₅ or Nb₂O₅ as a principal component on a surface tobe subjected to the direct bonding.
 7. The optical element according toclaim 1, wherein the thin film layer is a film formed on either the basesubstrate or the waveguide substrate in an atmosphere at a temperatureof not lower than 100° C.
 8. The optical element according to claim 1,wherein the thin film layer is a multilayer film including a metallayer, the metal layer being not arranged on a surface of the thin filmlayer on a side of the waveguide substrate, and the waveguide substrateis bonded with the thin film layer.
 9. The optical element according toclaim 8, wherein the metal layer is formed on a surface of thin filmlayer on a side of the base substrate.
 10. The optical element accordingto claim 8, wherein a surface of the metal layer on a side of thewaveguide substrate, and a surface of the waveguide substrate on a sideof the metal layer, are separated with a distance of not less than 50 nmtherebetween.
 11. The optical element according to claim 8, wherein adistance between the metal layer and a light-incident end surfaceexceeds 1 μm.
 12. The optical element according to claim 1, wherein thewavelength λ is one selected from 380 nm, 410 nm, 441.6 nm, 488 nm, 532nm, and 632.8 nm.
 13. The optical element according to claim 1, whereinthe waveguide substrate is made of a transparent crystal, and the thinfilm layer has a multilayer film structure made of a plurality ofdielectric materials having different refractive indices, the thin filmlayer being configured so that when the light having the predeterminedwavelength is incident perpendicularly on the waveguide substrate,lights reflected at the respective layers have phase differences suchthat the reflected lights do not cancel one another.
 14. The opticalelement according to claim 13, wherein the transparent crystal has alight transmissivity with respect to light with a wavelength in a rangecovering one selected from 380 nm, 410 nm, 441.6 nm, 488 nm, 532 nm, and632.8 nm.